Collision With Earth's "Little Sister" Created the Moon

astroengine writes The primordial planet believed to have smashed into baby Earth, creating a cloud of debris that eventually formed into the moon, was chemically a near-match to Earth, a new study shows. The finding, reported in this week's Nature, helps resolve a long-standing puzzle about why Earth and the moon are nearly twins in terms of composition. Computer models show that most of the material that formed the moon would have come from the shattered impactor, a planetary body referred to as Theia, which should have a slightly different isotopic makeup than Earth.

Comments

Re:Another explanation

[confused one]

Orbit is too circular for that solution to work.

Why different?

[kenwd0elq]

Why should the material composition of Theia have differed all that much from the Proto-Earth? They formed from the same planetary nebula, and at relatively similar distances from the Sun; shouldn't they have been similar in composition? And how can anyone state with any certitude, 4+ billion years later, how much of the merged Earth's crust was from Theia, and how much from the proto-Earth, and whether the lunar material was one, the other, or mostly mixed? It was a long time ago, and the Early Heavy Bombardment period would have stirred things up further. In fact, it's not unlikely that the Early Heavy Bombardment material was long-period debris from the original collision.

If Theia had formed substantially closer, or substantially farther away from the Sun, then the debris from the collision could hardly have remained close enough that the shards would coalesce to form the Moon. The differing orbital velocities would have seen to that.

Re:Earth's atmosphere was different

[TapeCutter]

Causing an environmental catastrophe at the time.

Yep, oxygen build up was a disaster for the cyanobacteria that created it and had reigned Earth for 3+ billion years. On the plus side the free oxygen enabled collagen to form, which is the substance that holds single cells together in multi-cellular organisms. We call that transition "The Cambrian explosion". The collision we are talking about occurred 3.5 billions years before the Cambrian explosion.